Food banks have started to issue specially prepared "kettle boxes" to clients who cannot afford to switch on their cooker to boil pasta or rice, in the latest sign of the cost of living crisis facing Britain's poorest.

The kettle boxes developed by volunteers from the Trussell Trust charity contain products that can be prepared by adding boiling water, such as instant soup, Pot Noodles, instant mash and just-add-water porridge, as well as staples such as crackers, cereal and tinned food.

For even more destitute clients, a "cold box" food parcel has been created, containing three days' worth of mainly tinned groceries that can be prepared without the need for heating or hot water.

The boxes, which the trust accepts do not meet the nutritional standards of its regular food parcels, were developed in response to clients who had refused to take basic items such as rice, pasta, tinned tomatoes and baked beans because they had too little credit in the electricity meter to cook them, or had been cut off by their gas supplier.

"We were absolutely astonished when this started to happen, and we were also really upset, " said Annette Smith, volunteer project co-ordinator of Morecambe Bay food bank in Lancashire. "Why is it happening? It's the old cliche: do I heat or eat?"

Morecambe Bay food bank is currently giving out on average two kettle boxes and two cold boxes a week, Smith said. The packages were not ideal, she admitted - it felt strange that a "cold box" client would not even be provided with tea bags or coffee (they get extra fresh juice instead).

Although the boxes do not quite meet the trust's nutritional guidelines for food parcels, "at least their [client's] bellies are full", Smith said. "You sleep better if you've had something to eat."

The food bank also helps clients deal with underlying causes of their problems, by referring them to advice agencies.

Smith said the food bank had provided a cold box as part of a package of help for one family whose electricity had been switched off for non-payment of a "stupid amount of money" totalling less than £100. Food bank volunteers subsequently arranged with the energy supplier to reconnect the supply and to have the outstanding sum repaid.

Chris Mould, executive chairman of the trust, said kettle boxes were "another example of how bad things have got" for low-income families. Nutrition was clearly an issue, he accepted, but added: "If you can't afford to turn the electricity on, then some food is better than no food."

Julie-Anne Wanless, manager of the St Andrew's community network in north Liverpool, which runs a Trussell Trust-affiliated food bank, said that initially kettle boxes were few and far between but the network had given out "quite a lot" in recent months.

Clients took kettle boxes and cold boxes for a variety of reasons: some had delayed or stopped benefits, while others were homeless and had just moved into bed and breakfast accommodation. Some clients simply could not afford the few pounds needed to charge up the meter.

Standard Trussell Trust food parcel contents are designed by nutritionists to contain three days' nonperishable food.

The trust's quarterly figures show 355,000 people received food parcels last year between April and the end of September – more than the total fed throughout 2012-13.

More than half its clients were referred as a result of benefit delays, sanctions or because of welfare cuts such as the bedroom tax, although ministers have insisted there is no causal link between welfare reform and charity food-aid growth.

The "heat or eat" dilemma - people having to choose whether to spend their meagre resources on food or heating their home – has emerged as a key poverty issue this winter.

According to the Real Life Reform housing association survey published last month, low-income social housing tenants in the north-west of England affected by welfare reform had reduced food spending to an average of £2.10 per person a day, partly to cope with the rising costs of gas and electricity.

Morecambe Bay food bank said it now provided "start-up" boxes containing tin openers, a tea towel and plates and cutlery for young people leaving care or young homeless families who had been moved into temporary accommodation.